Additional abbreviations found in the patent application are defined as follows:
BPSK Binary Phase Shift Keying
DFT Discrete Fourier Transform
FFT Fast Fourier Transform
LDPCC Low-Density Parity-Check Code
MAC Medium Access Control
PHY Physical
MCS Modulation Coding Scheme
PLCP Physical Layer Convergence Protocol
PSDU PLCP Service Data Unit
PPDU PLCP Protocol Data Unit
CBPS Coded Bits Per Symbol
STBC Space-Time Block Code
WLAN Wireless Local Area Network
HT-SIG High Throughput Signal
As wireless technology has advanced, a variety of wireless networks have been installed, such as cellular and other wireless networks. Some wireless networks are based upon the Institute of Electrical and Electronics Engineers (IEEE) 802.11 family of Wireless LAN (WLAN) industry specifications. Other wireless technologies are being developed, such as EEE 802.16, also referred to as WiMax, industry specifications. IEEE 802.16 defines the wireless air interface specification for wireless metropolitan area networks. A number of working groups are engaged in efforts to improve on this technology.
As is explained in, for example, IEEE802.16e™-2005, section 8.4.9.2.5.1, the LDPC code is based on a set of one or more fundamental LDPC codes. Each of the fundamental codes is a systematic linear block code. Using the method defined in section 8.4.9.2.5.2 (Code Rate and Block Size Adjustment), the fundamental codes can accommodate various blockcode rates and packet sizes. IEEE802.16e™-2005, sections 8.4.9.2.5.1 and 8.4.9.2.5.2 are incorporated by reference herein.
When using a small set of fixed length LDPC codes, a problem arises when attempting to utilize the LDPC codes in an OFDM transmission where the OFDM system has a fixed number of bits per OFDM symbol, for a given MCS, and a variety of data packet lengths. What can occur is that for a given MCS and a given packet length, there will exist some minimum number of OFDM symbols that allow for transmission of the data bits contained in a packet burst. The transmitter needs to then select a LDPC block size (i.e., codeword length) for the specified code rate and number of codewords to use in encoding of the information data bits. Then, the transmitter needs to map these code-bits onto the OFDM symbols using the modulation specified. Often times, however, the length of the packet payload data does match exactly the number of systematic bits (or integer multiple) of one of the codewords in the given set. When this occurs, the transmitter needs to determine the best possible choice of LDPC block size and the number of codewords to use.
Longer (larger) codewords benefit from better, more desirable coding gains relative to shorter (smaller) codewords (i.e., smaller codewords result in smaller coding gains).
Thus, if the transmitter uses too small of a codeword, then the transmission may suffer performance degradation relative to a longer codeword. On the other hand, if the transmission uses too large of a codeword, then the transmitter is required to (a) puncture too many codeword bits to fit in the finite period of transmission, resulting in a loss in link performance, or, (b) in order to avoid severely degraded decoding behavior due to excessive puncturing, the transmitter may need to send additional OFDM symbols beyond the minimum number of OFDM symbols that the packet may fit into, resulting in a longer transmission time for a given data rate. In either case, the transmitter uses more power than actually needed and may require a retransmission (possibly at a higher power) due to the additional OFDM symbols or performance loss. Thus, the communication system suffers throughput degradation due to this inefficient use of the air interface.
Various previous proposals have been directed to the foregoing problem(s). All such proposals known to the inventors, however, suffer from requiring the use of complicated algorithms or implementations when attempting to manage the performance versus air-time penalty trade-off. Some previous proposals would use a large set of codewords in an attempt to fit the codeword exactly or nearly exactly to the minimum number of OFDM epochs. Other previously proposed approaches suffer from poor performance due to the use of too small of a block size in situations when a larger block size may be used. Another drawback of the previously proposed approaches is the use of complicated block size switching algorithms that use a combination of reduced block lengths at the end of a packet, resulting in performance degradation. Yet another prior approach used a single codeword length, and did not attempt to achieve a more efficient packing with a multitude of codeword lengths.